Maternal Serum AFP

This is an outdated version of this FAQ. It was effective 09/12/2012 to 03/19/2013.

Question 1. My patient had a negative maternal serum AFP screen. What should I do next?

A negative screen means it is unlikely the fetus has an open neural tube defect. But a negative screen does not guarantee the birth of a healthy baby. The single AFP marker is not recommended as a screening tool for Down syndrome and other chromosomal abnormalities. If the patient has undergone a chorionic villus sampling (CVS) procedure during this pregnancy, with normal chromosomes, no additional follow up testing for Down syndrome is needed. If not, it is a standard of practice to offer a second trimester maternal serum screen that includes markers that screen for Down syndrome and trisomy 18.1 Second trimester screening options include the Triple screen (test code 7292), Quad screen (test code 30294) or Penta screen (test code 15934). Please contact 1-866-GENE-INFO to discuss the case with a genetic counselor.

Question 2. My patient’s result was screen positive for an open neural tube defect (ONTD). What should I do next?

A positive ONTD screen result means there is an increased risk for the fetus to be affected with a neural tube defect or other congenital abnormality. If the patient’s AFP is less than 3.5 MoM, if ultrasound dating confirms the gestational age and a singleton pregnancy, and if the gestational age is less than 18 weeks, testing a newly collected sample can be considered. If this second sample confirms an elevated AFP MoM or if a second AFP test is not performed, genetic counseling, ultrasound examination, and possible amniocentesis would be appropriate.

Note that underestimation of gestational age is one of the most frequent causes of an elevated MoM.

Question 3. The report indicates a different gestational age than what I determined. How is the reported gestational age calculated?

The gestational age is reported in decimal weeks; for example, 15 weeks 4 days is reported as 15.6 weeks. This is derived from the estimated date of delivery (EDD) and the collection date provided; it is an exact calculation by calendar days. Gestational wheels may be inaccurate by several days or more.

Question 4. When is it appropriate to change the gestational age or estimated date of delivery (EDD) on a maternal serum screen report?

It’s appropriate to change the gestational age or EDD when the data used for screening is substantially above or below that determined by ultrasound.

The earliest average ultrasound age (AUA) calculated should be used for dating purposes. AUA is most accurate when determined in the first trimester. Accuracy decreases with advancing maternal age. For example, AUA accuracy is ±7 days in the first trimester and ±10 days in the second trimester.

If first trimester AUA is available and the gestational age used for screening is within the AUA ±7 days, the gestational age should not be changed for screening purposes. Similarly, if second trimester (but not first trimester) AUA is available and the gestational age used for screening is within the AUA ±10 day range, the gestational age should not be changed for screening purposes. If the gestational age used for screening is outside the AUA range, it may be appropriate to change the gestational age used for screening.

If you want to change the EDD/gestational age used for a specific patient’s screening test, please contact your local Quest Diagnostics business unit or call 1-866-GENE-INFO. If the revised gestational age is between 15.0 to 22.9 weeks’ gestation, we can calculate and report a new NTD risk. If the revised gestational age is <15.0 weeks, we cannot calculate a new risk. Consider submitting a second specimen for screening, collected when the woman is between 15.0 to 22.9 weeks’ gestation (preferably 16 to 18 weeks). If the revised gestational age is >22.9 weeks’ gestation, we cannot calculate a new NTD risk, and a more accurate risk assessment cannot be provided.

The NTD detection rate is significantly lower at 14.0 to 14.9 weeks’ gestation than at 15.0 to 22.9 weeks’ gestation. Therefore, samples collected during the 14th week of gestation with an AFP value below 2.5 MoM (single gestation, nondiabetic) will be reported as screen negative without a patient-specific risk. For optimal NTD screening, collect samples when the woman is at 16 to 18 weeks’ gestation.

Question 6. Why is there no numerical NTD risk reported for women carrying twins or triplets?

AFP concentrations seen in NTD-affected and unaffected singleton pregnancies are well known. Therefore, an adjusted AFP MoM can be used to calculate an NTD risk in singleton pregnancies. AFP concentration data is insufficient, however, to allow similar calculations for twin and triplet pregnancies. Nevertheless, twin and triplet pregnancies can still be interpreted as screen positive or screen negative based on the AFP MoM.

Question 7. My patient has a family history of neural tube defect (NTD). What impact does this have on her NTD screen results?

Please call 1-866-GENE-INFO to discuss this case with a genetic counselor. Documentation of the abnormality in the family may enable a more specific risk assessment or indicate whether additional studies should be performed.

This FAQ is provided for informational purposes only and is not intended as medical advice. A physician’s test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.